The present invention relates to a method of fabricating thin film transistors to be used in pixel switches of a liquid crystal display and in a driving circuit therefore, and, more particularly, to a method of producing polycrystalline silicon of which a thin film transistor is comprised.
Today, liquid crystal displays to whose pixel switches insulated gate thin film transistors using amorphous silicon are adapted are mass-produced. Since the field-effect mobility of amorphous silicon is equal to or less than 1 cm.sup.2 /Vs, however, such a liquid crystal display has a difficulty in outputting a high-chroma image at a high speed.
As a solution to this problem, liquid crystal displays to whose pixel switches thin film transistors using polycrystalline silicon having a relatively high field-effect mobility are adapted are being put to a practical use. This polycrystalline silicon (hereinafter called "polysilicon") is produced by laser annealing which irradiates a laser beam from an excimer laser on amorphous silicon to crystallize it. It is known through experiments that the polysilicon shows a field-effect mobility of about 100 cm.sup.2 /Vs to 200 cm.sup.2 /Vs. Therefore, a liquid crystal display whose thin film transistors use polycrystalline silicon having a high field-effect mobility can output a high-chroma image fast.
It is known that the greater the particle size of polysilicon becomes, the higher the field-effect mobility of polysilicon gets. It is also known that the particle size of polysilicon depends on the energy density (fluence) of a laser beam which is irradiated on amorphous silicon by laser annealing. In other words, increasing the fluence of the laser beam can increase the particle size of polysilicon, thereby making the field-effect mobility higher.
When the fluence of a laser beam exceeds a certain value, the particles of polysilicon have a microcrystal size, so that the desired field-effect mobility cannot be acquired. The fluence of a laser beam should therefore be adjusted to fall within a range which can achieve the desired field-effect mobility. That is, the fluence of a laser beam is adjusted to range from a fluence F1 at which the minimum field-effect mobility needed for fast output of a high-chroma image can be obtained to a fluence F2 over which the particle size of polysilicon becomes a microcrystal size.
At present, however, the fluence margin from F1 to F2 is so narrow that a slight variation of the oscillation intensity of a laser causes the fluence to go off this fluence margin. This makes it difficult to acquire the desired particle size or the desired field-effect mobility. This results in a poor yield and an increase in the production cost at the time of producing the aforementioned high-performance polysilicon.